The present invention generally relates to laser systems and more particularly to a direct ultrashort laser system.
Recent ultrashort laser devices use optimization calculation approaches for pulse compression that do not require phase measurement, and that are able to characterize the phase after pulse compression, provided a calibrated pulse shaper is used. Pulse shapers such as an adaptive, pixelated SLM, traditional MEMS deformable membrane mirror, and a Dazzler acousto-optic crystal, and the related components, however, can be relatively expensive. Additionally, the ability to measure and correct the spectral phase of a laser becomes more challenging as the spectral bandwidth increases, such as for a sub-5 femtosecond duration pulse.
A challenge in ultrashort pulse characterization is the accurate measurement of abrupt (discontinuous) phase changes that may be introduced by pulse shapers and to some extent by dieletric optics. The curvature of the phase changes that can be measured by Multiphoton Intrapulse Interference Phase Scan (“MIIPS”) increases with the optical resolution of the pulse shaper being used. For example, a π-phase step to be accurately measured using 10 or 100 nm FWHM pulses and a 640 pixel pulse shaper should have a run longer than 0.15 or 1.5 nm, respectively. Another issue is that the minimum amount of chirp that can be measured increases for narrower bandwidths. For example, the uncertainty of a φ(ω) measurement for pulses spanning 10 or 100 nm FWHM would be ˜±500 or ±5 fs2, respectively. When MIIPS is implemented by using a spatial light modulator (“SLM”)-based pulse shaper, the maximum phase delay that can be introduced limits the measurable phase range. By phase wrapping and double passing the SLM, maximum delays of up to 1000 rad are possible. Furthermore, noteworthy improvements in laser pulse control are disclosed in U.S. Patent Publication No. 2006/0056468 entitled “Control System and Apparatus For Use With Ultra-Fast Laser,” and PCT International Application Serial No. PCT/US07/24171 filed on Nov. 16, 2007 entitled “Laser System Employing Harmonic Generation,” both of which were invented by Marcos Dantus et al. and are incorporated by reference herein.
In accordance with the present invention, a direct ultrashort laser system is provided. In another aspect of the present invention, a method of measuring laser pulse phase distortions is performed using passive optics, such as a prism-, grating- or prism-pair arrangement, and without requiring an adaptive pulse shaper or overlap between two or more beams. In another aspect of the present invention, a method for directly displaying the second derivative of the spectral phase distortions is performed without requiring a pulse shaper, overlap between two or more beams or an interferometer. In yet another aspect of the present invention, a system, a method of operating, a control system, and a set of programmable computer software instructions perform Multi-photon Intrapulse Interference Phase Scan processes, calculations, characterization and/or correction without requiring a spatial light modulator or such other expensive, adaptive pulse shaper. Furthermore, another aspect of the present invention employs methods, control systems and software instructions for calculating, measuring and/or characterizing an unknown phase distortion of a laser beam through use of the second derivative of the spectral phase and/or using a series of second harmonic spectra obtained under different chirp conditions to determine the spectral phase distortion. A further aspect of the present invention provides for automatic, real time and computer-controlled adjustment of optics associated with a femtosecond laser, stretcher and/or compressor to compensate for phase distortions based on calculations and/or measurements of the spectral phase distortions in ultrashort laser beam pulses without the use of a pulse shaper. Additionally, another aspect of the present invention allows for directly measuring the second derivative of an unknown phase.
The direct ultrashort laser system of the present invention is advantageous over conventional devices in that the present invention system is considerably less expensive to implement. For example, in certain embodiments, traditional optical hardware can be employed without expensive pulse shapers, or separate optical devices such as FROG or SPIDER, but will still allow for accurate measurement and/or characterization of otherwise unknown phase distortions within the laser pulse. This system can then be upgraded in a relatively easy manner by providing for higher level calculations of the measured phase distortions. Moreover, the system can be further upgraded to provide automatically controlled adjustments and compensation for the measured and/or characterized phase distortions to essentially eliminate undesired distortions. Accordingly, a low cost, easily upgradable and easy to practically implement system is achieved, while also providing excellent accuracy of results. For example, a non-adaptive and passive phase adjustable mirror is a reflective macroscopic optic which does not employ pixelation. By way of another example, a non-adaptive and passive pulse shaper has a single bendable optic. A further example of a non-adaptive and passive pulse shaper includes a manually adjustable actuator(s) that is not voltage driven. Such an optic is very efficient and is expected to return greater than 95% of the incident light (excluding other components such as gratings). In another example, a non-adaptive and passive pulse shaper has a single adjustable parameter. Furthermore, a piezoelectric actuator provides an additional exemplary non-adaptive and passive pulse shaper. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.